Rollup merge of #77209 - jyn514:fix-docs, r=petrochenkov

Fix documentation highlighting in ty::BorrowKind

Previously it looked a little odd: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.BorrowKind.html#variant.UniqueImmBorrow

Noticed this while reviewing rust-lang/rustc-dev-guide#894.

Author: jonas-schievink

compiler/rustc_middle/src/ty/mod.rs

@@ -682,25 +682,31 @@ pub enum BorrowKind {
     /// implicit closure bindings. It is needed when the closure
     /// is borrowing or mutating a mutable referent, e.g.:
     ///
-    ///    let x: &mut isize = ...;
-    ///    let y = || *x += 5;
+    /// ```
+    /// let x: &mut isize = ...;
+    /// let y = || *x += 5;
+    /// ```
     ///
     /// If we were to try to translate this closure into a more explicit
     /// form, we'd encounter an error with the code as written:
     ///
-    ///    struct Env { x: & &mut isize }
-    ///    let x: &mut isize = ...;
-    ///    let y = (&mut Env { &x }, fn_ptr);  // Closure is pair of env and fn
-    ///    fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
+    /// struct Env { x: & &mut isize }
+    /// let x: &mut isize = ...;
+    /// let y = (&mut Env { &x }, fn_ptr);  // Closure is pair of env and fn
+    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
     ///
     /// This is then illegal because you cannot mutate a `&mut` found
     /// in an aliasable location. To solve, you'd have to translate with
     /// an `&mut` borrow:
     ///
-    ///    struct Env { x: & &mut isize }
-    ///    let x: &mut isize = ...;
-    ///    let y = (&mut Env { &mut x }, fn_ptr); // changed from &x to &mut x
-    ///    fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
+    /// struct Env { x: & &mut isize }
+    /// let x: &mut isize = ...;
+    /// let y = (&mut Env { &mut x }, fn_ptr); // changed from &x to &mut x
+    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
     ///
     /// Now the assignment to `**env.x` is legal, but creating a
     /// mutable pointer to `x` is not because `x` is not mutable. We